1. Field of the Invention
The present invention relates to an optical information reproducing apparatus and an optical recording medium on which information such as image information is optically recorded. The present invention particularly relates to a recording medium capable of recording wideband signal such as high definition television signal, and to an apparatus for reproducing information from such record media.
2. Prior Art
Optical disks are formed therein with elongated circular pits arranged in a spiral around the center of the disk so that information is represented in terms of the presence and absence of pits. For example, an image signal and an audio signal are modulated onto an FM carrier frequency. The frequency-modulated signal is then supplied to an optical modulator which in turn outputs a modulated light. The output light of optical modulator is exposed to the surface of the optical disk to form pits therein in accordance with the image and audio information. In this manner, information is recorded onto the optical disk.
Optical video disk systems include CAV(constant angular velocity) system where an optical disk rotates at a constant angular velocity, and CLV(constant linear velocity) system where the length of a track traced by a light beams per unit time is constant. In the CAV system, the disk rotates at a speed of 1800 r.p.m. and one rotation makes one frame of picture. Since the sync signals are radially aligned in the CAV system, the CAV system is suitable for trick plays such as still pictures, slow motion pictures, and reduced shots. Meanwhile, the disk is rotated at a constant speed of about 11 m/s in the CLV system. The disk rotates at 1800 r.p.m. while information is read from the tracks closest to the center of disk. The rotational speed is gradually decreased as the information is read from the tracks far from the center of disk, reaching about 600 r.p.m. when the information is read from the outermost track. The innermost track is capable of recording one picture frame while the outermost track three frames. Thus, the CLV system is capable of recording information of twice as long time as the CAV.
In order to record and reproduce a high definition television signal such as Hi-Vision signal, a large amount of information needs to be recorded on an optical disk and it is therefore necessary to read a larger amount of information from the disk per unit time. That is, the optical disk must be capable of recording information over a wide frequency band. For implementing a wideband disk, smaller pits may be effective to increase the number of pits per unit length of track, so that more number of pits are read per unit time to reproduce information over a wide frequency band. In this, case, the minimum length of pits must be shorter than that of conventional disks. For good resolution of pits having such short pit lengths, a pickup must have a wide spatial frequency band. For the wide spatial frequency of pickup, it is necessary to use a light source having a shorter wavelength or to increase numeric aperture of an objective. Light sources having shorter wavelengths may be implemented by using gas lasers and other lasers that utilize non-linear optical elements in place of the conventional semiconductor lasers. However, this leads to large sizes of apparatus and increased costs. Larger numeric apertures of objective tend to cause errors in reproducing signals due to the uneven surface and tilt of the disk. Thus, the disk must have very uniform surface.
Another way of implementing wideband frequency characteristics of an optical disk may be to employ higher rotational speeds of disk. However, higher rotational speeds of disk require larger motors. Moreover, it is difficult to maintain accurate focusing of a light beam on the disk surface due to the tilt of disk and inclined rotational axis of disk drive. It is extremely difficult to accurately control the focusing of the light beam exposed on the disk surface at increased rotational speeds of disk.
Still another way of implementing wideband frequency characteristics of optical disk is to divide a wideband signal into a plurality of portions and to record them onto a plurality of tracks, respectively. Then, the recorded information is simultaneously read from the plurality of tracks, thereby implementing wideband frequency characteristics of a disk. However, in order to read the information from a plurality of tracks by means of a single pickup, the disk must be uniform over a large surface area. In addition, a plurality of light beams are needed to read information. For simple construction of a light source, a single light beam may be split into a plurality of beams. In this case, the respective beam may not have sufficient intensity to provide good signal-to-noise ratio.